Hitherto, in the steps of creating semiconductor devices made of SiC, when an epitaxial layer is formed on the surface of a SiC single-crystal substrate, there have been cases where the epitaxial layer is formed on the backside of the SiC single-crystal substrate when gas that is generated passes around the substrate. In order to remove the epitaxial layer formed on the backside thereof, a step of polishing the backside of the SiC single-crystal substrate (hereinafter, referred to as the backside polishing step) has been carried out. Meanwhile, this backside polishing step also has an effect of lowering the on-resistance of a semiconductor device by thinning the substrate.
However, when the backside polishing step is performed, there have been cases where local thickness variation (referred to as the LTV: Local Thickness Variation) becomes large in the SiC single-crystal substrate. Moreover, the LTV is the difference between the maximum value and the minimum value of the thickness within a unit surface of the SiC single-crystal substrate, and for example, in an area of 10 mm2 can range from 2 to 10 μm.
When the value of the LTV is larger than the focus depth (1 μm or so) of a stepper, a problem occurs in that even in the case of formation of a fine pattern of a semiconductor region using the stepper, a desired fine pattern cannot be formed, and thus it is impossible to create a semiconductor device having desired characteristics.
The semiconductor device includes, for example, a Schottky barrier diode. This Schottky barrier diode is a diode in which a Schottky barrier φb generated by the junction of a metal and a semiconductor is used, and has a feature that the forward voltage drop is low and the switching speed is high, compared to PN junction diodes. However, when a Schottky metal having a small Schottky barrier φb is used in order to further lower the forward voltage drop, there is also a defect that the reverse leakage current (hereinafter, referred to as the leakage current) is large and the reverse withstanding voltage is low. In addition, since the Schottky barrier diode is a unipolar device, there is also a defect that resistance to the forward surge current is low too. The semiconductor region made of a fine pattern is formed, whereby it is possible to improve a reverse withstanding voltage property, and to improve resistance to the forward surge current.
Particularly, in the Schottky barrier diode in which a PN junction region and a Schottky junction region are combined, when a gap of the PN junction region becomes wide because a fine pattern cannot be formed, the application of the reverse voltage increases an electric field of the Schottky junction region and increases the leakage current. In addition, since a Schottky metal having a small Schottky barrier φb cannot be used, there is a case where the forward voltage drop is increased. Further, the ratio of the area occupied by the Schottky junction region in the entire electrode is decreased, and thus there is a case where the voltage drop when the current flows in the forward direction is increased, leading to an increase in power loss.
For this reason, in the semiconductor device such as the Schottky barrier diode, it is essential to thin the substrate and to form the semiconductor region made of a fine pattern in order to improve the semiconductor properties.
PATENT DOCUMENT 1 relates to a method of manufacturing a semiconductor device chip and a semiconductor device, and discloses a wafer thinning step of forming a junction terminal after processing the backside thereof.
In addition, PATENT DOCUMENT 2 relates to a SiC single-crystal substrate for creating a semiconductor device and a method of manufacturing the same, and discloses a step of forming a SiC single-crystal substrate from a SiC single crystal ingot. In addition, PATENT DOCUMENT 3 relates to a method of manufacturing a SiC single-crystal substrate, and discloses a step of removing an altered portion of the SiC single-crystal substrate by etching in which reaction gas is used. Furthermore, PATENT DOCUMENT 4 relates to a light-emitting diode in which a current block layer is provided between a light-emitting section and a surface electrode. PATENT DOCUMENTs 2 to 4 disclose that dry etching of SiC is performed at an etching rate of several μm/h or so.
However, even when these methods are used, it is impossible to solve the aforementioned problems.